A known method for the production of cyanohydrins is the base-catalyzed addition of hydrogen cyanide to carbonyl compounds, such as aldehydes and ketones. Cyanohydrins containing 3 to 6 carbon atoms are valuable intermediates. Acetone cyanohydrin is particularly important because it can be converted to methacrylate esters, and also to dimethylhydantoin derivatives and azobis(isobutyronitrile).
For the production of end products from cyanohydrins, the latter must often be available in very pure form, in general above 97%. Important impurities in crude cyanohydrins are unreacted reactants, that is hydrogen cyanide and low-boiling carbonyl compound, since the formation and dissociation of cyanohydrins set up an equilibrium. While cyanohydrin formation is base-catalyzed, the crude cyanohydrins are stabilized before work-up or purification by addition of an acid.
It is known to produce acetone cyanohydrin by continuously feeding the reactants acetone and hydrogen cyanide into alkaline catalyzed reaction circuits or into reactors formed as flow tubes. An equilibrium is established between the starting materials and acetone cyanohydrin at the reaction temperature--generally in the range 0.degree.-50.degree. C. In order to achieve high conversion in a practicable time, the reaction is usually carried out in several stages at decreasing temperature. The catalyst is then neutralized and, if necessary, separated, and the crude acetone cyanohydrin stabilized by making it weakly acidic. This permits the unconsumed starting materials to be separated under vacuum at temperatures up to 80.degree. C. As far as possible, the reactants which are distilled off are re-circulated to the cyanohydrin reactor--cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. (1985) pages 91-92.
In the previously known processes, conventional apparatus, such as liquid ring pumps, vapor pumps or Roots blowers are used to provide a vacuum during the purification of the crude cyanohydrin. These systems, however, have significant disadvantages: in order to generate the vacuum required, multistage systems are necessary. These are more expensive and often require repairs. Such systems must further be followed by complicated installations for waste water treatment and/or waste gas washing. In addition, it was necessary to have intermediate condensation of the unconsumed reactants before their return to the cyanohydrin reactor.